polyamorphism

300px|thumb|Pressure–temperature phase diagram, including an illustration of the liquid–liquid transition line proposed for several polyamorphous materials. This liquid–liquid [[phase transition would be a first order, discontinuous transition between low and high density liquids (labelled 1 and 2). This is analogous to polymorphism of crystalline materials, where different stable crystalline states (solid 1, 2 in diagram) of the same substance can exist (e.g. diamond and graphite are two polymorphs of carbon). Like the ordinary liquid–gas transition, the liquid–liquid transition is expected to end in a liquid-liquid critical point. At temperatures beyond these critical points there is a continuous range of fluid states, i.e. the distinction between liquids and gasses is lost. If crystallisation is avoided the liquid–liquid transition can be extended into the metastable supercooled liquid regime.]]

300px|thumb|Schematic of interatomic pair potentials. The blue line is a typical Lennard-Jones potential|Lennard-Jones type potential, which exhibits the ordinary liquid–gas critical point. The red line is a double well type potential, which is proposed for polyamorphous systems. The grey line, is a representative of the soft core square well potentials, which in atomisitc simulations exhibit liquid–liquid transitions and a second critical point. The numbers 1 and 2 correspond to the 1st and second minima in the potentials.